@Possum:

In PVE? No, never. The only fight where a Shadow Priest was remotely useful compared to the much more powerful Mages and moderately more powerful Warlocks was on a burn fight with constant pulsing AOE damage that Vampiric Embrace could heal, like Hakkar in Zul'Gurub and... well, pretty much Hakkar. And even then, it could be argued that either the extra healing of a Holy Priest or the damage output of pretty much any DPS class would be superior to the mixed, inferior bag of the poor Shadow Priest. That having been said, there were a few die-hards like myself who refused to 'see the light' and still insisted on raiding as Shadow.

In PVP? We did a little better. Not MUCH better, but better than in PVE. Back in the days before Cloak of Shadows, Shadow Priests had a better than average life expectancy against Rogues thanks to our DOTs and the slow effect of Mind Flay. Even so, we were underpowered compared to Warlocks, who had everything we had, plus no cooldown on fear, more powerful DOTS, and a pet. More often than not, we were seen as healers with DOTs, and more often than not, that's how we played.

Not that we had it the worst of the class/spec combinations (anyone remember Vanilla Subtlety? or Arcane?) - but we had it pretty bad. Thanks to tier design at the time, if we wanted decent gear for raiding, we had to participate in the endless PVP grind to get at least our Blue PVP set (I almost made it to Champion, got sick, lost three ranks, and gave up in disgust).

All those people who look back with fondness on Vanilla? Rose-colored glasses.
Seriously, there was a lot of fun stuff in Vanilla, but damned near everything that made it fun was made better with the successive expansions. Anyone who says differently either wasn't there or isn't remembering all of the really, really bad details.